Bovine tracheal myosin kinase (M.W.=160,000) and human platelet myosin kinase (M.W.-100,000) have been purified to apparent homogenity. We have investigated the phosphorylation of these two myosin kinases by the catalytic subunit of cAMP-dependent protein kinase and by the cGMP-dependent protein kinase. We have found that like turkey gizzard, (M.W.=130,000) platelet and tracheal myosin kinases can be phosphorylated by the catalytic subunit of cAMP-dependent protein kinase, although the molecular weight differs among these three enzymes. This phosphorylation decreased the affinity of myosin kinase to bind calmodulin. cGMP-dependent protein kinase can also phosphorylate these myosin kinases. However, phosphorylation by cGMP-dependent protein kinase had either no or only a slight effect on myosin kinase activity. We are now studying the effect of another type of Ca2+ dependent protein kinase (protein kinase C) on myosin kinase and myosin.